Supercritical fluid extraction (SFE) is used to analyze a sample. It offers a relatively rapid, simple and inexpensive technique to do the same. The basis of SFE is that the sample being analyzed is introduced to a fluid which is above its critical temperature and pressure. Such a fluid is otherwise known as a supercritical fluid. Supercritical fluids have favorable diffusivities and viscosites providing for good mass transfer characteristics. Their strength can be easily controlled, and since most are gases at ambient conditions, are easily workable. These are but a few of the advantages of supercritical fluids.
Typically, an SFE system is comprised of a pump which pumps the supercritical fluid to an extraction vessel where analytes are extracted from a sample matrix. The analytes are then transported to a collection device and the supercritical fluid, which is now no longer supercritical is vented. See "Analytical-Scale Supercritical Fluid Extraction" by S. B. Hawthorne, Analytical Chemistry, Vol. 62, No. 11, Jun. 1, 1990, pp. 633-642 and "Analytical Supercritical Fluid Chromatography and Extraction" by M. Lee and K. Marbides, Chromatography Conferences, Inc., 1990, incorporated by reference, for a general discussion of SPE.
As is apparent from the fact that a supercritical fluid is above its critical temperature and pressure, there needs to be an element of the SFE system which causes the supercritical fluid to be converted to a fluid at, for instance, ambient conditions. In general, some form of a flow restrictor is utilized between the extraction vessel and the collection vessel to accomplish this requirement. An SFE system is described in "Characterization of Sulfur Compounds in Spices Using SFE-GC-AED" by W. S. Miles and B. D. Quimby, American Lab, July 1990; and "Evolution of Coupled Supercritical Fluid Extraction--Cryogenic Collection--Supercritical Fluid Chromatography (SFE-CC-SFC) for Quantitive and Qualitative Analysis" by M. Ashrof-Khorossani, M. Kumar, D. Koebler, and G. Williams, Journal of Chromatographic Science, Vol. 28, Nov. 1990, pp. 599-604. In Miles, et al., supra, there is used a needle type restrictor which controls the flow of the fluid with analyte by varying the distance of the needle tip to an opening.
However, it is recognized that in order to maximize the transfer of analyte and fluid from high pressure to lower pressures as true a turbulent flow is desired thereacross. A restrictor which has steep internal tapers (decreasing from 50 .mu.m to less than 1 .mu.m within less than 1 mm, leading to a pinhole opening produces nearly turbulent flow and is very desirable. See "Linear Velocity Control in Capillary Supercritical Fluid Chromatography by Restrictor Temperature Programming" by A. Berger and C. Toney, Journal of Chromatography, 465 (1989) pp. 157-167. Also, see "Performance of Capillary Restrictors in Supercritical Fluid Chromatography" by R. Smith, J. Fulton, R. Peterson, A. Kopwa and B. Wright, Anal. Chem. 1986, 58, 2057-2064; and "Integral Pressure Restrictor for Capillary SFC" by E. Guthaie and H. Schwartz, Journal of Chromatographic Science, Vol. 24, June 1986, 236-241 which discuss the general importance of restrictors.
Also, see U.S. Pat. No. 3,827,859 which discloses a diaphragm arrangement for various pressure gradients thereacross in a flame ionization detector.
The present invention provides an orifice in an off-line SFE system that allows essentially all analyte to be gathered. The off-line feature of the present invention allows the analyte gathered to be taken to a remote location for desired testing.